Lecture 12: Apoptosis II

Question 1

Signalling regulation of the intrinsic pathway- Akt

Answer 1

Akt (kinase activated by
numerous growth factor
receptor signals) can inactivate
Bad and downregulate Bim
and Puma expression through
the TF, FOXO3a
* Akt also prevents cleavage of
Bid from extrinsic pathway

Question 2

Signalling regulation of the intrinsic pathway-p53

Answer 2

• p53 protein levels can
  increase in healthy cells
  upon cellular stress
• p53 induces expression of
  BAX, PUMA, and NOXA,
  and inactivation of Bcl-2

Question 3

Extrinsic pathway of apoptosis

Answer 3

• Death receptor pathway
• Induced by external signalling – ligand binding to a specific
  membrane receptor
• Death receptors/ligands are members of the TNF (tumour
  necrosis factor) family of proteins – TNF-alpha, TRAIL, FasL
• Two ways to activate death receptor pathway signalling:
• Increase ligand expression
• Increase receptor expression

Question 4

Ligands and their receptors

Answer 4

TRAIL (TNF-related apoptosis inducing ligand)–DR4 AND DR5

TNF– TNFR1

Fas Ligand (FasL)— Fas (APO-1/CD95)

Question 5

Key steps in the extrinsic pathway

Answer 5

• Ligand-receptor binding
• Receptor trimerisation
• Recruitment and formation of Death
  Inducing Signalling Complex (DISC)
• DISC recruits and activates the initiator
  caspase (procaspase8, sometimes
  procaspase 10). DISC triggers selfcleavage, thereby activating, caspases.
• Active initiator caspases then activates
  executioner caspases

Question 6

Death receptors

Answer 6

• Receptor trimerisation brings
  3 Death Domains (DD)
  together
• This aggregation allows for
  recruitment of FADD (Fasassociated death domain),
  which forms the Death
  Inducing Signalling Complex
  (DISC)

Question 7

From DISC to Death

Answer 7

• Active caspase 8 (initiator
  caspase) can lead to direct
  activation of caspase 3 and
  activation of BID, which
  promotes MOMP

Question 8

Extrinsic pathway - anoikis

Answer 8

• Anoikis – “homeless” – anchorage
  independent
• Lack of stimulation from extracellular matrix
  (through integrin) turns off pro-survival
  signalling (PI3K-Akt pathway or NK—κB)
• Stimulates caspase 8 activation

Question 9

Direct immune cell killing through
extrinsic apoptosis pathway- FasL

Answer 9

• Fas expression can be increased
  due to p53 activation
• FasL can be membrane bound on
  nearby cell or truncated-soluble
  form
• FasL on cytotoxic T cell binds to
  Fas on tumour cell
• Stimulates extrinsic pathway in
  tumour cell and perforin and
  granzyme release from cytotoxic T
  cell

Question 10

Cleaning up the mess

Answer 10

• Phagocytic cells are
  recruited to the area
• PCs recognize
  phosphatidylserine on
  the apoptotic cell
  surface
• Engulfment and
  degradation